In my opinion every six weeks upgrade is a standard on rapid cycle. So for me Firefox was doing very well in adding features and squash out the issues and bugs.
Firefox 6 already sees bump in traffic soon after debut
Firefox 6 just launched last week, and new research already sees the latest version of Mozilla’s browser accounting for a significant fraction of traffic.
Ladies and gentleman, Firefox 6.01 is out. This explains the reason
for this update.
http://blog.mozilla.com/security/2011/08/29/fraudulent-google-com-certificate/
Removed trust exceptions for certificates issued by Staat der Nederlanden (see bug 683449 and the security advisory)
Resolved an issue with gov.uk websites (see bug 669792)